An ionic electrode method, which makes use of an ion-selective electrode, has been known for detecting ions in a sample solution. By the ion-selective electrode herein is meant such electrode which detect the potential difference corresponding to the activities of anions and cations contained in the solution that is the thermodynamically compensated concentrations of the anions and cations, and which is provided with a membrane of an ion-sensitive material selectively sensitive to certain predetermined ion species.
However, with the ionic electrode method, it is necessary to use an ion-selective electrode and a reference electrode separately. Also, in many cases, an internal liquid solution is employed for each of the electrodes. As a result, the electrodes tend to be increased in size and, in addition, the sample being measured needs to be used in a larger quantity. If the internal liquid solution is not used and a lead wire is directly attached to the ion-sensitive membrane to take out the electrical potential, the response time tends to be prolonged or fluctuated and hence a stable measurement cannot be achieved. If the internal liquid solution is used, attention must be directed to the state of replenishment or maintenance of the internal liquid solution.